1. Field of the Invention
This invention relates to read-only memories (ROM's), and more particularly to a starting product from which a ROM may be produced. Moreover, this invention also relates to a method of forming ROM starting products, and a method forming ROMS therefrom.
2. Description of the Prior Art
ROM's are well-known as are methods of making them. One method of making a ROM, of particular interest herein, involves the selective blowing out or fusing of fusible links which constitute a portion of metallic conductive layers in an electrical element so as to prevent selected ones of the elements from operating. The electrical element may be a resistor, a diode, or a unipolar or a bipolar transistor, usually of a multilayered configuration.
Common today are MOSFETS, using negative logic. The fusible links are usually portions of the conductive layers which are unable to carry as much current as the remainder of the layer. Generally speaking, the inability of a MOSFET to operate is thought of as storing a "0" therein. MOSFETS which are able to operate exhibit a "1" when appropriate voltages are applied thereto.
MOSFETS and other multilayered electrical elements are produced by photolithographic techniques which involve successive and repetitive deposition masking, and selective etching steps. The elements may each include a doped region in a body of semiconductor, various electrodes respectively contacting or being associated with appropriate sites of the body and/or region. When a ROM is to be produced, the individual elements are often interconnected by conductive layers or buses to form a matrix thereof. The conductive layers are often a continuation of, and are formed by the same steps as, the electrodes of the elements.
Fusible links in such structures have been formed in the conductive layers and/or the electrodes in numerous ways.
One way has been to deposit the conductive layers in part over small bumps or steps formed in an insulating layer to thin out the metal at this point. Another way has been to form part of the conductive layer over a dissolvable area, such as a photoresist bump, which is later dissolved to suspend the part in air thus decreasing the heat dissipative effects thereat. These prior art methods are discussed in U.S. Pat. No. Re. 28,481 which itself relates to formation of fusible links by deposition of thin, precisely defined and delineated fuse metal layers through an appropriate mask. Such fuse metal deposition is in addition to a subsequent, separate step of depositing the conductive layers, the two being selectively brought into contact through apertures formed in an insulating layer separating the fuse metal and the conductive layers.
The first two approaches, while desirable because they add no extra steps to the usual photolithographic sequence, produce links which fuse unpredictably or not at all. The third approach adds an extra metal layer (the fuse metal) and an additional insulating layer and therefore adds additional steps to the usual photolithographic sequence.
Another approach is to decrease the cross-sectional area of selected parts of the busses after their formation. See U.S. Pat. Nos. Re. 28,481 and 3,529,299. Such decrease is effected by a necking down of the bus, usually by an extra masking step followed by etching. Beside the extra processing steps required, this approach leads again to unpredictable fusing.
However formed, the fusible links are subsequently blown out or fused by selectively applied currents.
Prior art references disclosing other types of ROM and their manufacture are U.S. Pat. Nos. 3,582,908 (a resistive ROM) and 3,245,051 (a diode ROM). These patents involve the formation of a matrix of components with selected crosspoints being blown out or destroyed by destructive current being passed therethrough. Such currents are often unpredictable and may, or may not, fuse the desired crosspoints. Moreover, the use of such currents may adversely affect components other than those which it is desired to affect.